I've made the change you're asking about. I was lucky enough to find a NOS Duo-Track fork ( a mid-level fork) from the mid-90's with the elastomers still in excellent condition. It had the right type of steerer (1" threaded) and the thread length and position was a perfect fit for my bike. (This is almost as hard to find as hitting the lottery). I work on bikes for a hobby so I installed it myself, no bike shop expenses. My total cost was only $50.
The final results were less than expected. The old bike was designed for a rigid fork and the new fork changed the geometry substantially. With the suspension fork the bike handled logs and rough terrain much better than before. BUT, the bike had no stability. When cornering, the bike needed a wider path, when climbing, if I hit a stall, the bike wanted to fall down.
So even if you manage to make this change on a budget, don't expect it to be anywhere near a new low end bike from an LBS.